Recent tendency of microelectronics requires a smaller, lighter and thinner galvanic cell having a high energy density according to using the cell in the form of inclusion in electronic devices and unification with electronic elements and circuits as represented by the memory back-up powder source for various electronic devices. In the field of primary galvanic cell, small light galvanic cells such as lithium cell have been already used in practice. However, their fields of application are restricted. Here, the secondary galvanic cell by using a non-aqueous electrolytic solution to enable a smaller and lighter galvanic cell has been noticed as a galvanic cell replacing conventional lead cells and nickel-cadmium cells. However, no galvanic cell satisfying practical performances such as cycle characteristics and self-discharge characteristics of electrode active material has been found.
Here, we, inventors, have investigated the preparation of a thin galvanic cell (sheet cell having a thickness of 100 to 500 .mu.m per unit cell) which is small and light and has a high energy density by using a thin membrane of an ionic conductive polymer. When a thin membrane of the above-mentioned ionic conductive polymer is used, it has become to be problems that the preparation of a thin membrane of metal lithium having a quality fully matching it is technically difficult to some extent and that the manufacturing process of the galvanic cell becomes complex. In addition, when it is used as a secondary galvanic cell, a problem has also occurred that the use of metal lithium is restricted because of the difficulties such as formation of lithium dendrite and passivation of the surface.
Thus, lithium metal-containing alloys such as lithium-aluminum, lithium-lead and lithium-tin alloys have been extensively investigated. However, as represented by lithium-aluminum alloys, these alloys are low in strength and hence the electrode is cracked or pulverized by repeated charge and discharge to be failed in the improvement of cycle characteristics.
As other methods for preventing formation of lithium dendrite, selection of the electrolyte salts and improvement of the separator have been tried. Though the prevention of lithium dendrite has been tried by laminating polypropylene nonwoven fabrics and glass fiber nonwoven fabrics conventionally used as a separator, it gives no substantial solution.
Accordingly, now in many research laboratories, those utilizing intercalation or doping phenomenon of layer compounds have been especially investigated as the electrode active material. As these cause no complex chemical reaction theoretically during the electrochemical reaction of charge and discharge, a very excellent charge-discharge cycle performance can be expected.
On the other hand, a liquid electrolyte, particularly an organic electrolytic solution containing dissolved ionic compound has been generally used as the electrolyte for the galvanic cells utilizing electrochemical reaction and the electrochemical devices other than the galvanic cell such as an electric double layer capacitor and an electrochromic element. However, a liquid electrolyte tends to cause leakage of the liquid from the parts, elution and evaporation of the electrode material and hence it causes problems including lack in long-term reliability and scattering of the electrolytic solution during the sealing process.
In order to improve the liquid leakage resistance and the storage stability, it is required to use an ionic conductive polymer having a high ionic conductivity, that is a solid electrolyte. However, when the conventional ionic conductive polymers are used, they may cause problems including complex manufacturing processes of the galvanic cell and restriction in use of metal lithium as mentioned above. Thus, it was difficult to provide a small light cell which is excellent in long-term reliability and safety and also has a high performance and a high energy density.
For example, the known solid electrolytes include a polymer solid electrolyte manufactured by a combination of a trifunctional high polymer having a terminal acryloyl-modified alkylene oxide chain, a low molecular alkylene oxide copolymer, polyvinyl chloride and an electrolyte salt, etc. (Japanese Laid-Open Patent Publication No. 177409 of 1991) and a solid electrolyte manufactured by a combination of a terminal acryloyl-modified alkylene oxide chain, an inorganic ionic salt and an organic solvent such as propylene carbonate, etc. (Japanese Laid-Open Patent Publication No. 94501 of 1988). However, these had problems in the volume and the mechanical strength, and a small light cell having a high energy density could not be prepared with them.
Here, an object of the present invention is to improve an ionic conductive polymer and thus to provide a galvanic cell which is excellent in long-term reliability and safety and also has a high performance and a high energy density though small and with no fear of liquid leakage.